Hitherto, aluminum (Al) alloy films have been used for interconnections of display devices, typified by liquid-crystal displays. As progress is made in an increase in the size and quality of display devices, problems of signal delay and power loss due to high wiring resistance are manifested. Thus, copper (Cu) having lower resistance than Al has been receiving attention as a wiring material. Al has an electrical resistivity of 2.5×10−6 Ω·cm, whereas Cu has a low electrical resistivity of 1.6×10−6 Ω·cm.
However, it is impossible to ensure sufficient adhesion of Cu to a gate insulating film (typical examples thereof include Si oxides and Si oxynitride, such as SiOx and SiON). That is, Cu has low adhesion to an oxygen-containing insulator layer, thereby disadvantageously causing delamination. Furthermore, Cu has low adhesion to the oxygen-containing insulator layer, so it is difficult to subject Cu to wet etching in order to form an interconnection shape. A main component of a glass substrate is a Si oxide, so there is the same problem as the gate insulating film. To improve adhesion to the glass substrate, various techniques are reported.
For example, PTLs 1 to 3 each disclose a technique for improving adhesion by interposing a high-melting-point metal layer composed of, for example, molybdenum (Mo) or chromium (Cr), between a Cu interconnection and a glass substrate. In these techniques, however, the number of steps of forming the high-melting-point metal layer is increased to increase the production cost of a display device. Furthermore, Cu and the high-melting-point metal (e.g., Mo), which are different metals, are stacked, so that corrosion may occur at the interface between Cu and the high-melting-point metal during wet etching. Moreover, the different metals have different etching rates, thereby disadvantageously failing to form the cross section of an interconnection into a desired shape (e.g., a shape with a taper angle of about 45° to about)60°. In addition, the electrical resistivity (12.9×10−6 Ω·cm) of the high-melting-point metal, such as Cr, is higher than that of Cu, thereby disadvantageously causing signal delay and power loss due to wiring resistance.
PTL 4 discloses a technique in which nickel or a nickel alloy and a polymer-based resin film are interposed as an adhesion layer between a Cu interconnection and a glass substrate. In this technique, however, the resin film may be degraded in a high-temperature annealing step in the production of a display (e.g., liquid-crystal panel) to reduce adhesion.
PTL 5 discloses a technique in which copper nitride serving as an adhesion layer is interposed between a Cu interconnection and a glass substrate. However, copper nitride itself is not a stable compound. Thus, in this technique, N atoms may be released as N2 gas in a high-temperature annealing step in the production of a display (e.g., liquid-crystal panel) to degrade an interconnection film, thereby reducing adhesion.